The present invention relates to a process for the preparation of a Li.sub.2 O--Al.sub.2 O.sub.3 --SiO.sub.2 (LAS) ceramic sintered body, and in particular to a process for preparing a ceramic LAS sintered body by adding lithium fluoride to a calcined LAS ceramic powder as sintering agent and sintering at a temperature of 950.degree. C.-1250.degree. C.
Li.sub.2 O--Al.sub.2 O.sub.3 --n SiO.sub.2 ceramics (n is 2,4, 6 and 8 where n=2 is eucryptite, n=4 is spodumene, n=6 is lithium orthoclase and n=8 is petalite), due to having very low thermal expansion coefficient, have been widely used as thermal shock-resistant ceramics in high temperature applications to prevent the cracks originated from thermal shocks. Among them, .beta.-spodumene with a zero-approaching thermal expansion coefficient, 9.times.10.sup.-7 /.degree.C., and superior mechanical strength, chemical resistance and dimensional stability, has been applied in the manufacture of components of heat exchangers, frames of microwave ovens, and base plates of electromagnetic furnaces etc.
Conventionally, low thermal expansion coefficient ceramic materials, like LAS ceramic materials, are fabricated by glass-forming technique, a technique traditionally used in the manufacture of glass-ceramics. This technique includes melting the raw material at a temperature of above 1500.degree. C., fabricating the molten material into desired shapes and heat treating the shaped ceramics. The homogeneity of the produced ceramics usually is not satisfactory, and the scale of energy consumed in melting is high. Furthermore, in the manufacture of some high performance products, in particular multiple-component ceramic products, the products manufactured by glass-forming technique cannot meet the desired requirements.
In recent years, sol-gel processing has been widely used in the manufacture of ceramics because of inherent advantages compared to conventional processing. For example, the high surface area of dried gels can result in very high reactivity which permits low temperature processing. By starting with well mixed solutions or sols, chemical homogeneity even on the molecular scale can be obtained. However, the gels prepared by the sol-gel processes can shrink non-uniformly during the drying and calcinating, resulting in partial collapse, hardening and agglomeration of the gels. Accordingly, grinding or ball milling of the dried gels is necessary. This however will inevitably reduce the purity and the forming property of the resulting powders, render the sintered bodies porous, undensified, and thus affect the mechanical properties of the final products.
A technical report entitled "Preparation of .beta.-spodumene powder by sol-gel process and properties of sintered bodies" (Hidehiko Kobayashi et al Journal of Ceramic Soc. Japan, 98(7), 1990, 703) discloses a .beta.-spodumene powder prepared from alumina sol, silica sol, and lithium nitrate solution by sol-gel process.
Sintered bodies are then obtained from the powder. The relative density of the sintered bodies was 96% when sintered at a temperature of 1000.degree.-1100.degree. C. The average thermal expansion coefficient of the sintered bodies was 1.4.times.10.sup.-6 /.degree.C. The porosity of the sintered bodies was still high and the pore size was about 1-10 .mu.m.